1. Field of the Invention
The present invention relates to a peak suppressor and more particularly to a peak suppressor built in a wireless transmitter.
2. Description of the Related Art
In the field of wireless digital communication, a technology to improve transmission amplifier efficiency involving peak suppression of a transmission target signal prior to amplification by a transmission amplifier is conventionally known. When a peak-to-average power ratio (PAPR) of an input signal to the transmission amplifier is reduced by peak suppression, output back-off of the transmission amplifier decreases, and amplifier efficiency improves.
FIG. 15 is a graph illustrating the relationship between input/output characteristics of the transmission amplifier and the peak of each signal. As shown in FIG. 15, the signal in the lower portion of the figure having an operating point A as an average power has a large back-off, and hence efficiency of the transmission amplifier is reduced. On the other hand, the signal in the upper portion of the figure is obtained by clipping a signal that has an operating point B as an average power, a waveform similar to that of the signal in the lower portion of the figure and a power larger than a given power, i.e., it is a signal subjected to peak suppression. Since the PAPR of the signal is reduced by peak suppression in this manner, the back-off is restrained, thereby enhancing the efficiency of the transmission amplifier. Various peak suppression modes have been proposed.
FIG. 16 is a block diagram of a wireless transmitter having a conventional clipping mode peak suppressor. As shown in FIG. 16, a conventional clipping mode peak suppressor 1 has an amplitude calculator 2, a comparator 3, a suppression coefficient calculator 4, a selector 5, a delay unit 6, and a multiplier 7. The amplitude calculator 2 calculates an amplitude of a transmission target signal generated by a digital signal generator 8. If i and q respectively represent amplitudes of a signal i and a signal q of the transmission target signal, then an amplitude of the transmission target signal is obtained based on [√{square root over ((i2+q2))}]. The comparator 3 compares the amplitude obtained by the amplitude calculator 2 with a preset threshold value Vth.
The suppression coefficient calculator 4 calculates [Vth/√{square root over (i2+q2))}] as a value required to suppress a peak of the transmission target signal. The selector 5 selects a suppression coefficient based on a result of the comparison by the comparator 3. The selector 5 selects the value obtained by the suppression coefficient calculator 4 if the amplitude obtained by the amplitude calculator 2 is larger than the threshold value Vth [Vth/√{square root over ((i2+q2))}>Vth], and selects 1 if not. The delay unit 6 delays the transmission target signal for a period equivalent to the time required from amplitude calculation of the transmission target signal generated by the digital signal generator 8 to selection of the suppression coefficient.
The multiplier 7 multiplies the transmission target signal output from the delay unit 6 by the suppression coefficient selected by the selector 5. Therefore, the peak suppressor 1 outputs a peak suppressed transmission target signal when the amplitude obtained by the amplitude calculator 2 is larger than the threshold value Vth, and outputs the transmission target signal as it is when not. A digital/analog (D/A) converter 9 converts the transmission target signal output from the peak suppressor 1 into an analog signal, a mixer 10 multiplies the analog signal by a local carrier wave from a local oscillator 11, and a resulting signal is input to a transmission amplifier 12. The signal amplified by the transmission amplifier 12 is transmitted via an antenna 13.
FIG. 17 is a block diagram of a wireless transmitter having a conventional windowing function mode peak suppressor. As shown in FIG. 17, a windowing function mode peak suppressor 16 has a windowing function generator 17 between the suppression coefficient calculator 4 and the selector 5 in the peak suppressor 1 depicted in FIG. 16. That is, the peak suppressor 16 is configured to apply a windowing function generated by the windowing function generator 17 to a value obtained by the suppression coefficient calculator 4. Peak suppression distorts a signal in both the clipping mode and the windowing function mode, thereby leading to deterioration in reception quality. That is, a reduction in the PAPR due to peak suppression and the reception quality have a trade-off relationship. Therefore, the degree of peak suppression must be determined so as to satisfy the reception quality set for each system.
A transmitter that controls for signal level adjustment of a multi-carrier signal obtained by combining multiple carriers is known. This transmitter includes a peak suppressing unit that detects the presence/absence of a peak based on a sum of the power levels of respective input carriers and outputs carriers obtained by suppressing power levels of respective carriers so as to reduce the total power level below a predetermined peak threshold value when a peak is detected. The transmitter further includes an input power calculator that calculates an average power level with respect to the respective carriers before input to the peak suppressing unit and an output power calculator that calculates an average power level with respect to the respective carriers after output from the peak suppressing unit; a monitoring unit that outputs level control information required to control signal levels of respective carriers output from the peak suppressor based on the average power level calculated by the input power calculator and the average power level calculated by the output power calculator; and a level adjusting unit that adjusts the levels of respective carriers based on corresponding level control information. For an example of such a transmitter, refer to Japanese Patent Application Laid-open No. 2004-166245, claim 3, paragraph 0025.
However, the aforementioned conventional peak suppression technologies are all technologies for suppressing signal peak in a single system, and do not perform system specific signal peak suppression for multiple systems where each system has a different required reception quality. The Japanese Patent Application Laid-open No. 2004-166245 does not clearly explain or suggest that peak suppression is performed for multiple systems specific to the different reception quality requirements of each system.
Therefore, a problem arises with the conventional technologies when the same frequency band is used to perform services in systems. For example, under the Third Generation Partnership Project (3GPP), standardization of a 3GPP long-term evolution system (LTE system) is currently in progress.
The LTE system uses the same frequency band as that in widely prevalent third-generation (3G) systems. Therefore, both the LTE system and the 3G system coexist in the same frequency band at the start of a service of the LTE system. In such a case, provision of separate transmission amplifiers for each of the systems, the LTE system and the 3G system, in a transmitter is inefficient. Accordingly, a configuration where a common transmission amplifier is provided for both of the systems and signals of both of the systems are input to the transmission amplifier in a multi-carrier combined state is desirable.
However, the LTE system and the 3G system have defined reception qualities that are different from each other. Therefore, similar to the conventional technologies, by a configuration in which peak suppression is performed on a combined multi-carrier signal obtained by combining the signals of both systems and the resulting signal is input to the transmission amplifier, peak suppression is performed in compliance with the system having a higher required reception quality, i.e., the LTE system. Accordingly, weak peak suppression is effected with respect to the system having a lower required reception quality, i.e., the 3G system, resulting in a problem that PAPR cannot be sufficiently reduced.
To at least solve the above problems in the conventional technologies, it is an object of the present invention to provide a peak suppressor that can perform an appropriate degree of peak suppression on signals of multiple systems according to differing reception quality requirements of each system.